Introduction: Frequent users of Emergency Departments (ED; ≥ 5 ED visits in the previous 12 months) often cumulate somatic, psychological and substance use problems. Providing a Case Management (CM) intervention may reduce their number of ED visits and improve their quality of life. However, there is limited knowledge about the implementation process of CM. This study aimed to identify the facilitators, barriers and needs encountered during the CM implementation in the context of a larger study that aimed to implement CM in EDs thorough French-speaking Switzerland. Methods: Mixed-method research were conducted with ED staff involved in CM implementation completed a questionnaire (N = 31) and took part in semi-structured interviews (n = 23) assessing implementing influencing factors. Quantitative data were analyzed with Kruskal-Wallis one-way tests, whereas qualitative data were subject to an inductive content analysis. Results: Using several dimensions of the RE-AIM framework, ED sites were split into two groups: more and less successful sites. Quantitative Findings showed a significant association between the perception of acceptability (x2 (2, n=30) =14.39, p=.045), appropriateness (x2 (2, n=30) =15.36, p=.032) and feasibility (x2 (2, n=30) =17,25, p=.016) of CM intervention measure and the success of the CM implementation. Qualitative Findings revealed four main facilitators: 1) Direct hierarchy support and flexibility; 2) Exchange with colleagues; 3) Supervision by the research team; and 4) Motivation. Lack of resources was an unanimously mentioned barrier. Participants identified the following needs to enable CM implementation: official and protected time for the project and at least two team members with complementary skills (e.g.: somatic, psychiatric and social). Conclusions: The way the CM implementation was perceived by the professionals contributed to the success of the implementation process. That said, beyond their convictions, human and logistical resources were found as crucial. Findings highlight that influencing factors for implementation are at individual and structural levels.